Bonding of lead wires between a microcircuit chip and the lead frame in which the chip is mounted for coupling to external circuitry is generally accomplished by "ball/wedge" bonding. According to this technique a lead wire or bonding wire 11 is held in a capillary tool 12 so that the lead wire 11 projects beyond the end of the tool as shown in FIG. 1. The capillary tool 12 forms part of a ball bonding machine in which the tool is appropriately mounted and positioned over the metallized die pad 14 of an integrated circuit chip or other substrate. As shown in FIG. 1 a ball 15 of metal is formed at the end of the lead wire or bonding wire 11 by melting for example with a gas flame torch 16 such as an hydrogen flame torch. This procedure is sometimes referred to as "flame-off".
After solidification, the metal ball 15 at the end of the wire is brought into intimate contact with the metallized die pad 14 as shown in FIG. 2. A bond is formed typically by thermocompression bonding applying a specified force and temperature for a specified period of time. Metallic welding and diffusion combine to form this basic bond. Alternatively, ultrasonic bonding or other form of energy may be used. The capillary tool 12 and substrate 17 are then moved relative to each other for bonding of the wire at another location such as for example a lead frame finger 18. At this location a wedge bond between the lead wire 11 and finger 18 is generally formed and the lead wire or bonding wire 11 is severed below the bonding tool. In this manner a lead wire connection is established between the metallized substrate of a chip and the lead frame for coupling to external circuitry.
Further background on ball bonding of lead wires can be found in the article "Evaluating Wire Bond Quality" by Steven W. Hinch and Donald R. Cropper in the February 1981 issue of Semiconductor International.
Ball bonding is the preferred method for welding lead wires to the die pad of integrated circuit chips because the ball can tolerate a greater range of bonding parameters without weakening the wire and furthermore, the lead wire can be led in any direction from the symmetrical weld. A number of problems are encountered in ball formation however which have generally limited its application to the use of relatively expensive gold lead wires and bonding wires. The primary difficulty in applying the ball bonding method to, for example copper wire and aluminum wire occurs during ball formation. The tip of the wire is melted either by a hydrogen gas torch as shown in FIG. 1 or by arc discharge between the tip of the wire and a suitably placed electrode. For arc formation capacitor discharge has been used typically about 500 volts from a two microfarad capacitance. However, during ball melting and formation the copper or aluminum or other reactive metal wire oxidizes and the resulting oxide film prevents or interferes in the subsequent ball weld to the surface pad. Oxidation also prevents uniform quality ball formation. As a result the ball bonding technique has generally been limited to the use of gold wires.
Ball bonding has been applied to aluminum wires by the Welding Institute, Abbington Hall, Abbington, Cambridge, England CBI 6AL as appears in the article "Ultrasonic Ball/Wedge Bonding of Aluminum Wires" by C. J. Dawes, K. I. Johnson, and M. H. Scott in the proceedings of the European Hybrid Micro Electronics Conference 1979, Ghent. This article appears under the same title by C. J. Dawes and M. Weldl in the January 1979 issue of the Welding Institute Research Bulletin, on pages 9 through 14. According to the Welding Institute method a hydrogen flame is not used for ball formation or flame-off because of oxidation. Rather electric discharge is used by means of a capacitor discharge technique and with an argon gas shield. A capacitor discharge ball forming attachment for aluminum wire was incorporated by the Welding Institute on the Kulicke and Soffa Industries, Inc. 472 ball bonding machine. A capacitor of selected size is discharged between the aluminum wire and an electrode in the presence of inert gas.
A disadvantage of the Welding Institute method is that the capacitor discharge generated arc results in considerable variation in the energy delivery to the end of the bonding wire for melting and ball formation. The capacitor discharge also results in considerable variation in the ionization and arc formation between the wire and complementary electrode.
Referring to FIG. 3A a detail is provided of the tip of a prior art capillary tool. The tip of the capillary tool 12 shows the central capillary channel 20 in which the bonding wire is held so that it extends below the tip of the tool through the conical flare 21. The lead wire or bonding wire may be typically in the order of 25 micrometers in diameter. In the previous electronic flame off methods such as that of the Welding Institute, capacitor discharge to form the ball offers little control over the energy delivered. As a result the wire may melt into the capillary 20 and the capillary must be pushed out or cleaned. The single irregular pulse discharge from a capacitor further results in variability of the pulse of energy. Yet another disadvantage of the irregular discharge is that excess energy may be delivered beyond what is precisely required for melting the desired volume of metal at the end of the wire. This excess energy causes undesirable oxidation and oxide formation on the ball metal. Prior art attempts at electronic flame off have generally resulted in ball formation of non uniform quality.
An example of a ball bonding machine which uses capacitive discharge of a single irregular pulse for ball formation is the Dage Precima AB-10 "Aluminum Spark Ball System". This system is identified by Model no. 83-9461 available from Dage Precima International in Colchester, United Kingdom. This machine uses a series of capacitors which may be switched in and out of parallel to control or vary the quantity of charge at the same voltage. However, the capacitors are fully discharged in a single irregular pulse. These capacitors are controlled by an SCR which stays on until the capacitive discharge current falls to an extremely small holding level.